1. Field of the Invention
This invention relates to inhibiting deposition of aluminum hydroxide in felts of a papermaking system. More particularly, this invention relates to inhibiting aluminum hydroxide deposition in a felt in a press section of a papermaking system wherein the felt is prone to such deposition and the felt is conditioned by showering with an aqueous medium.
2. Description of the Prior Art
When fresh, untreated shower water is utilized for conditioning of press felts on papermaking machines producing paper or paperboard from pulp suspensions containing alum (aluminum sulfate), and the resulting pH of the white water/fresh water mixture in the felts falls in the approximate range of 4.8-8.0, a sufficient quantity of insoluble aluminum hydroxide can precipitate alone or in conjunction with other substances from the white water and cause the felts to become prematurely filled and compacted. This results in reduced paper machine productivity and/or the need to prematurely remove the felts from the machine, the latter leading to increased operating costs and increased lost production time.
This problem of aluminum hydroxide deposition has been overcome historically by treatment of the shower water with strong acids such as sulfuric or phosphoric acid fed from bulk supply or in the form of specialty felt conditioning products. The purpose of the strong acid is to reduce the shower water pH to a level at which aluminum hydroxide will not precipitate, which is typically around a pH range of 4.0-4.5. However, this approach has several disadvantages. For example, when the shower water is especially alkaline, large quantities of acid or acid-based felt conditioning product is required which can be both costly and dangerous. Also, the acidic shower water causes accelerated corrosion of the shower piping, nozzles, and other parts of the felt conditioning system. Additionally, recent studies conducted by the present inventors have shown that the pH range of approximately 5.5-7.0 is more optimum for the performance of the most effective surfactants utilized as felt conditioning agents to inhibit felt filling and compaction caused by tacky wood pitch components or rosin size.
Effective chemical conditioning of a press felt helps to reduce the rate of felt compaction, maintain maximum felt absorbency, and prolong the felt's useful operating life. A felt must be kept clean of filling materials that adhere to the felt fibers and accumulate in the felt structure. These filling materials not only impede the flow of water through the felt, but also create adhesion between felt fibers, thus increasing the tendency for the felt's structure to compact and lose absorption capacity. Effective felt conditioning is particularly important for high synthetic fiber content felts which are seldom removed because they are worn out. They are generally removed because they become filled and compacted to the point where adequate absorption capacity is lost.